Development of a Non-Equilibrium Plasma Coupled Rapid Compression Machine for Advanced Ignition Studies

Abstract

A plasma-coupled rapid compression machine (PRCM) was developed to investigate plasma-assisted combustion (PAC) under elevated-pressure and elevated-temperature conditions representative of end-of-compression (EOC) engine environments. The facility enables controlled comparison of autoignition, conventional spark ignition (CSI), and nanosecond pulsed discharge (ns-PD) ignition. The experimental approach combines time-resolved pressure measurements, synchronized electrical diagnostics (voltage, current, and energy deposition), and high-speed CH* chemiluminescence imaging to quantify early flame evolution and flame propagation. Baseline autoignition experiments established mixture reactivity under identical thermodynamic conditions. Methane exhibited single-stage ignition with monotonic reduction in ignition delay with increasing temperature, while n-butane showed two-stage ignition with a distinct negative temperature coefficient (NTC) regime, consistent with low-temperature chain-branching chemistry. Ignition experiments at 10 bar and 664 K were performed using CSI and ns-PD systems. Flame development time (FDT) and flame propagation time (FPT) were extracted from pressure traces to separate kernel formation from subsequent flame growth. CSI results show that ignition improvement is primarily governed by early breakdown energy and voltage rise rate, while additional dwell time increases post-kernel energy deposition with limited impact on FPT, indicating diminishing returns once a self-sustaining kernel is formed. Nanosecond pulsed discharge ignition was investigated using three geometries to understand the contribution of distinct glow and spark regimes towards ignition enhancement. Experiments were performed over pulse repetition frequencies (1–100 kHz), voltages (24–29 kV), and pulse counts up to 100 pulses, with selected cases held at constant total energy (~95 mJ) to isolate inter-pulse coupling effects. Compared to CSI, ns-PD ignition reduced both FDT and FPT, with performance strongly dependent on discharge morphology. High-speed CH* chemiluminescence imaging at 25 kHz was used to resolve ignition kernel evolution and flame propagation. Results show that spark-containing discharges produce larger initial kernels due to rapid thermalization, while glow-dominated discharges exhibit slower initial growth but accelerated late-stage flame propagation. Area-averaged emission reveals an initial post-discharge decay in CSI and spark systems due to relaxation of excited species, whereas glow-dominated discharges show smoother evolution due to sustained radical production. Lean-burn experiments demonstrated improved ignition robustness for ns-PD systems relative to CSI, extending the practical lean limit to φ = 0.45 with improved cycle-to-cycle variability. Collectively, these studies advance the experimental understanding of plasma-assisted combustion by establishing the relationships between discharge morphology, energy deposition, inter-pulse coupling towards combustion enhancement. The resulting database provides a comprehensive benchmark for the development, validation, and refinement of plasma-assisted combustion models and PAC-specific chemical kinetic mechanisms applicable to practical high-pressure combustion systems.